1. Field of the Invention
The present invention relates to a phase shift mask capable of enhancing the resolution of a transfer pattern by utilizing the interference action of a light by a phase shifter and a phase shift mask blank to be a material thereof, and more particularly to a halftone type phase shift mask and mask blank.
2. Description of the Related Art
As one of the examples of a super-resolution technique of photolithography in recent years, there raised a phase shift mask. In particular, since a halftone type phase shift mask can be subjected to a pattern processing comparatively easily during the fabrication of the mask, it has widely been used mainly for the formation of a contact hole. Furthermore, there has recently been progressed an application to a cyclic pattern and an isolated pattern such as a line and space (L & S) in a DRAM.
As shown in FIG. 27, the halftone type phase shift mask comprises, on a transparent substrate 2, at least a light transmitting section 7 and a halftone phase shifter section 5 having a light semi-transmitting property and a phase shift function and can be roughly divided into a monolayer type and a multilayer type in respect of the structure of the halftone phase shifter section 5. The monolayer type is currently a mainstream because of the easiness of a workability and the halftone phase shifter section is constituted by a monolayer film comprising MoSiN or MoSiON in most cases. On the other hand, in the multilayer type, the halftone phase shifter section is constituted by a combination of a layer for mainly controlling a transmittance and a layer for mainly controlling a phase shift amount, and it is possible to independently control a spectral characteristic represented by the transmittance and the phase shift amount (phase angle).
On the other hand, it can be expected that the wavelength of an exposure light source (the wavelength of an exposed light) will be reduced from an existing KrF excimer laser (248 nm) to an ArF excimer laser (193 nm), and furthermore, to an F2 excimer laser (157 nm) with the microfabrication of an LSI pattern in the future. With such a reduction in the wavelength of the exposure light source, the range of choice of the material of the halftone phase shifter section to satisfy a predetermined transmittance and phase shift amount tends to be reduced. With the reduction in the wavelength of the exposure light source, moreover, a material having a high light transmitting property is required at a conventional wavelength. As a result, there is a problem in that an etching selectivity with a quartz substrate is reduced in patterning. A multilayer type (two-layer film) halftone phase shifter has advantages that a phase difference and a transmittance can be controlled by a combination of the two-layer films and a material can easily be selected and that such a material as to play a part in the etching stopper of an upper layer can be selected as a lower layer (JP-A-2001-174973). Therefore, the multilayer type (two-layer type) halftone phase shifter has been developed.
At the step of inspecting the pattern appearance of a mask which is fabricated, generally, an optical inspection is carried out. In general, the wavelength of an inspecting light is different from that of an exposed light and a light having a longer wavelength than the wavelength of the exposed light is usually selected as the wavelength of the inspecting light. More specifically, a light in the vicinity of a wavelength which is longer than the wavelength of the exposed light by one generation or more (a light in the vicinity of the conventional wavelength of the exposed light) is often used as the wavelength of the inspecting light. The reason is that the guarantee of precision in the inspection of the pattern appearance is stricter than the guarantee of precision in the transmittance of the exposed light. For a pattern appearance inspecting apparatus, a transmission type defect inspecting apparatus is used (for example, KLA300 series).
In order to obtain sufficient precision in a pattern appearance inspection in the phase shift mask, it is necessary to take a transmission contrast of the patterned halftone phase shifter section and the light transmitting section. For this reason, it is necessary to reduce the transmittance of the inspecting light in the halftone phase shifter section. More specifically, the film of the halftone phase shifter section is designed such that the light transmittance of an i ray having a wavelength of 364 nm to be the inspecting light is approximately 40% or less in the pattern appearance inspection utilizing the transmitted light of a KrF (a wavelength of 248 nm) compatible halftone type phase shift mask.
A technique for reducing a transmittance for the wavelength of an inspecting light has been described in JP-A-7-168343 gazette. This gazette has disclosed that a phase shifter section has a two-layer structure including a monolayer film (a phase adjusting layer) such as MoSiO or MoSiON which is known as a monolayer type halftone phase shifter and a transmitting film (a transmittance adjusting layer) having the small wavelength dependency of a transmittance in a combination with the monolayer film and a desirable transmittance can be thus obtained for both an exposed light (KrF excimer laser) and an inspecting light (488 nm).
With the progress of a reduction in the wavelength of the exposed light source described above, however, a halftone phase shifter having a higher light transmitting property than that at a conventional wavelength is required. Therefore, there is a problem in that a light transmittance for the wavelength of the inspecting light is increased. In such a material having a high light transmitting property, particularly, the rate of an increase in a transmittance for a change in a wavelength toward the long wavelength side tends to be increased. Consequently, it is much harder to reduce the light transmittance for the wavelength of the inspecting light down to a predetermined range.
More specifically, for example, it is necessary to further increase the transmittance of a phase difference adjusting layer at a conventional wavelength to cope with a reduction in the wavelength of the exposed light source down to an ArF excimer laser (193 nm), and furthermore, an F2 excimer laser (157 nm) in a mask having a two-layer type half tone phase shifter section comprising the phase difference adjusting layer and the transmittance adjusting layer. In the case in which the transmittance of the phase difference adjusting layer is thus increased, the transmittance for the inspecting light cannot be sufficiently reduced by the transmittance adjusting layer if priority is given to the maintenance (control) of the transmittance for the exposed light. If the thickness of the transmittance adjusting layer is increased to reduce the transmittance for the inspecting light, the transmittance for the exposed light cannot be ensured. In other words, there is a problem in that it is hard to adjust both the maintenance of the transmittance for the exposed light and the reduction in the transmittance for the inspecting light with the thickness of the transmittance adjusting layer.
On the other hand, for the mainstream of an existing halftone type phase shift mask, a film is designed in such a manner that the transmittance of the exposed light in the halftone phase shifter section approximates to 6%. A high transmittance has been required for a further increase in a resolution, and it is said that a transmittance of 15% or more will be required in the future. Also in that case, there is a problem in that the light transmittance of the halftone phase shifter section for the inspecting light is more than 40% and sufficient precision in an inspection cannot be obtained in the inspection of the appearance of a pattern utilizing a transmitted light in the same manner as in the above-mentioned case.
Furthermore, as for a photomask, a reflecting optical system has conventionally been used in a defect inspection. In addition, recently, the reflecting optical system as well as a conventional transmitting optical system have been utilized in the inspection of the pattern appearance of the mask. For this reason, a reflecting contrast of a light semi-transmitting section and a substrate at the wavelength of the light source of an apparatus has been required for obtaining a sufficient inspection sensitivity.
Furthermore, in the case in which a halftone type phase shift mask of a so-called tritone type having a light transmitting section, a light semi-transmitting section and a shielding section provided on the light semi-transmitting section is to be inspected, a reflecting contrast is required for each of the light transmitting section/the light semi-transmitting section, the light semi-transmitting section/the shielding section, and the light transmitting section/the shielding section. More specifically, R1<R2<R3 is required, wherein light reflectances at the wavelengths of light sources in the light transmitting section, the light semi-transmitting section and the shielding section are represented by R1, R2 and R3, respectively.
In order to detect a small defect having a size of 0.1 μm or less and a finer pattern, a wavelength has been reduced from an i-ray (365 nm) to the vicinity of 200 to 300 nm to be a DUV (deep ultraviolet) region in the light source of an inspecting apparatus. The present time is in a transition period. Therefore, it is desirable that a mask and a mask blank should have an optical characteristic corresponding to both inspecting apparatuses of an i-ray light source and a DUV light source. For the DUV light source, 266 nm and 257 nm are being investigated.
In a two-layer halftone type phase shift mask comprising a transmittance adjusting layer and a phase adjusting layer, a light reflectance fluctuates greatly within a range including the wavelength described above. For this reason, there is a problem in that a sufficient reflecting characteristic cannot be obtained at an inspection wavelength (a DUV wavelength) even if it can be obtained at another inspection wavelength (for example, an i-ray wavelength). Consequently, the design of a halftone film is to be changed corresponding to the wavelength of the light source of the inspecting apparatus, which imposes a great burden on development.
Moreover, it is a matter of course that a transmittance and a phase shift amount are adjusted to have desirable values for an exposed light having a short wavelength such as an ArF excimer laser (193 nm) or an F2 excimer laser (157 nm), and it is preferable that a reflectance at the wavelength of the exposed light should also be somewhat low.
For an item required for a photomask in the next generation, the wavelength of an exposed light is to be reduced in respect of an optical characteristic, and furthermore, quality is to be enhanced, that is, defects and foreign substances are to be lessened. For this purpose, it is essential that a system for inspecting a photomask and a photomask blank should be maintained.
In the defect and foreign substance inspection for the photomask and the photomask blank, an optical type inspecting apparatus is mainly used. In general, the light source of the inspecting apparatus uses a light having a longer wavelength than that of the wavelength of an exposed light. Examples of a defect inspecting apparatus for the photomask blank include a mask blank defect inspecting apparatus M-1320 manufactured by Laser Tech Co., Ltd., and the light source of an inspecting apparatus represented by the same apparatus mainly has a wavelength of 488 nm. In order to maintain precision in the defect and foreign substance inspection for the photomask and the photomask blank, accordingly, it is necessary to set a transmittance and a reflectance for an inspecting light within a certain range.
In a two-layer type halftone phase shift mask blank comprising a transmittance adjusting layer and a phase adjusting layer according to the conventional art, however, there is a problem in that the wavelength dependency of a reflectance is very great and a transmittance and a reflectance for an inspecting light do not satisfy the range by only the control of a transmittance and a reflectance for an exposure wavelength in some cases. For example, the minimum value of a fluctuation in the reflectance for a change in a wavelength reaches an inspection wavelength to be used for the defect inspection of the mask blank. As a result, the reflectance for the inspection wavelength is too low to recognize the reflection intensity of a clean mask blank surface (on which a foreign substance is not present) so that the defect inspection cannot be carried out.
In addition, a light source having the same wavelength is not always used in an apparatus for inspecting the photomask and an apparatus for inspecting the photomask blank. In the inspection for the photomask, furthermore, different light sources are used in an inspection for the defect of the shape of a pattern and an inspection for a foreign substance and a defect in some cases. Under present situations, there are inspecting apparatuses using light sources having wavelengths 364 nm, 266 nm and 257 nm in the inspection for the shape of the pattern of the photomask. On the other hand, light sources having wavelengths of 364 nm and 488 nm are mainstream in an inspection for the foreign substance of the photomask, and the light source having the wavelength of 488 nm is used in most of the defect and foreign substance inspections for the photomask blank. Such a difference is made by a variation in the developing speed of each inspecting apparatus and a period for introducing an apparatus user or a difference in the type and range of a defect and a foreign substance to be objects between the photomask and the photomask blank. Therefore, a transmittance and a reflectance for the inspecting lights of the photomask and the photomask blank are to be adjusted corresponding to various wavelengths. It is hard to fabricate the photomask blank and the photomask in the conventional art. There is caused a situation in which an optical characteristic for the inspecting light having the wavelength of 488 nm is not satisfied if the adjustment is carried out to have an optical characteristic to satisfy the inspecting light having the wavelength of 364 nm, for example.